1. Field of the Invention
The present invention relates to a binder resin for a toner used for development of an electrostatic image in electrophotography, electrostatic printing and the like, and a toner containing the binder resin for electrophotography. Also, the present invention relates to a binder resin for a toner and a toner containing a polyester resin.
2. Background of the Related Art
With the progress of office automation, the demand for electrophotography-based copying machines and printers has increased rapidly and performance requirements for such equipment have become higher.
There is a generally used method which comprises forming an electrostatic latent image on the photo-sensitive material, then developing the image using a toner, transferring the toner image onto a fixing sheet such paper or the like, and heat-pressing the transferred toner image using heat roll (a heat roll fixing method). In this heat roll fixing method, in order to achieve enhanced economical efficiency in powder consumption and increased photography speed, and to prevent papers from being curled, a toner superior in fixing properties is needed, which can be fixed at a lower temperature. On the other hand, there is such a problem, a so-called offset phenomenon, in the heat roller fixing method that since a toner is contacted with the surface of the heating roll in the melt state of the toner, the toner is transferred by adhering on the surface of the heating roll, and the transferred toner is transferred again on the next sheet to be fixed to contaminate it. Even without causing this offset phenomenon, high performance requirements are demanded as one of the important performance requirements for the toner. Meanwhile, as copiers and printers have come to employ a higher speed, high performance requirements in electrified portions have increased. That is, a higher durability is required for a toner and a longer-term stability in printing is needed.
In the conventional technique, there has been proposed a method to improve the molecular weight and molecular weight distribution of a binder resin for a toner. Concretely, it has been tested to have a binder resin of a low-molecular weight and to lower the fixing temperature. However, the melting point has been lowered owing to the lowered molecular weight whereas the cohesive force of a resin was also deteriorated simultaneously. Therefore, an offset phenomenon occurs to the fixing roller. In order to prevent such a problem, its has been attempted to use a binder resin having a wide molecular weight distribution by mixing a high molecular weight resin and a low-molecular weight resin, or, further to crosslink the high molecular weight portion of the binder resin or the like. With this method, however, it is difficult to satisfy the fixing properties owing to the increased viscosity of the resin. As a binder resin for a toner using the above method, in general, a styrene acrylic resin (e.g. 55-6895B and JP 63-32180B, U.S. Pat. No. 5,084,368, etc), a polyester resin (e.g. JP61-284771A, JP62-291668A, JP07-101318A, specification of U.S. Pat. No. 4,833,057), or a polyol resin (e.g. JP11-189647A, etc.) is mainly used.
However, the toners obtained by these techniques cannot be fully effective in high-speed copiers or small copiers in which the amount of heat from the heat fixing roll is not sufficiently transmitted. That is, when a polymer having a high weight-average molecular weight or a crosslinked polymer is used in order to prevent the offset phenomenon, the viscosity of a resin increases so that the fixing properties are deteriorated.
In order to achieve these requirements, there is a method to add paraffin waxes, low-molecular weight polyolefin or the like as a mold release agent to a toner. In JP49-65232A, JP50-28840A, JP50-81342A or the like is disclosed a technique to use a styrene type binder resin. However, block resistance and the development properties have been deteriorated whereas an offset phenomenon has been improved. It is also confirmed that the effects of a mold release agent are small when applied to a polyester resin and the developing agent is rapidly depleted when its amount is increased.
In order to provide a developing agent having sufficient fixing properties and offset resistance and being capable of providing high image quality of copying images, it needs to give a sufficient electrophotographic features to the developing agent. Many methods have been attempted heretofore in order to achieve high image quality and high resolution of copying images; however, no perfect methods could be obtained heretofore to fully overcome the above defects.
Meanwhile, in recent years, an increase in population has increased the amount of energy and has depleted resources. In connection therewith, resources saving, energy saving, recycling of resources, etc. have been claimed. As to polyethylene terephthalate (PET) bottles, individual municipalities have started a recycling activity, and PET bottles have come to be utilized as clothes or containers. In addition, development of newly recycled items is desired. Thus, a toner using a polyester resin has become a strong candidate.